Reinforced and conductive resin compositions comprising polyolefins and poly(hydroxy carboxylic acid)

ABSTRACT

A resin composition comprising a polyolefin, carbon nanotubes and poly(hydroxy carboxylic acid). The invention also covers a process for preparing a resin composition comprising a polyolefin, carbon nanotubes and poly(hydroxy carboxylic acid) by (i) blending a poly(hydroxy carboxylic acid) with carbon nanotubes to form a composite (ii) blending the composite with a polyolefin. The use of poly(hydroxy carboxylic acids) as a compatibilizer to blend carbon nanotubes into polyolefins is also claimed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of PCT/EP2008/061058, filed Aug. 25,2008, which claims priority from EP 07114921.5, filed Aug. 24, 2007.

TECHNICAL FIELD OF THE INVENTION

The present invention is concerned with obtaining polyolefincompositions containing nanotubes.

BACKGROUND OF THE INVENTION

It has been known for many years that blending fibres into polymers cansignificantly improve the mechanical properties of the polymers inquestion. Long fibres made of materials such as metal, glass or asbestos(GB 1179569 A) have been used to this effect. Boron, silicon carbide andeven carbon fibres have been developed for this purpose. The initiallydeveloped carbon fibres had diameters of several tens of microns andlengths in the order of millimeters. They were quite light and despitethis had impressive mechanical properties, displaying Young's moduli inthe range of 230 to 725 GPa and strengths in the range of 1.5 to 4.8GPa. Carbon fibres, also known as carbon nanofibres, having higheraspect ratios, have also been prepared having even smaller diameters ofabout 100 nm and lengths up to 100 microns, Young's moduli in the rangeof 100 to 1000 GPa and strengths in the range of 2.5 to 3.5 GPa.

However, the most recent development, resulting from the discovery ofBuckminsterfullerene (C60), is the carbon nanotube having unprecedentedphysical and chemical properties. A single wall carbon nanotube (SWNT)is a one-atom thick sheet of graphite (called graphene) rolled up into aseamless hollow cylinder which can have a diameter of the order of 1 nmand lengths of up to several millimeters. The aspect ratio can thuspotentially reach values of several millions. Multi-walled carbonnanotubes (MWNT) have also been developed, which are concentric arraysof single-walled carbon nanotubes (also known as the Russian Dollmodel).

With Young moduli of up to 5 TPa and mechanical strengths even greaterthan 70 GPa, carbon nanotubes have great potential to replaceconventional carbon fibres as polymer reinforcements.

Carbon nanotubes are also extremely light and have unique thermal andelectronic properties. Depending on how the graphene sheet is rolledi.e. the relationship between the axial direction and the unit vectorsdescribing the hexagonal lattice, and depending on the diameter, on thenumber of walls and on the helicity, the nanotube can be designed to beconducting or semi-conducting. Carbon nanotubes of high purity areextremely conductive. In theory, pristine carbon nanotubes should beable to have an electrical current density of more than 1,000 timesgreater than metals such as silver and copper. This is because noscattering of charge occurs as it travels through the tube, resulting inwhat is known as ballistic transport of the charge. Nanotubes may thusbe added to an electrically insulating polymer to produce conductiveplastics with exceedingly low percolation thresholds as described in WO97/15934.

As for thermal properties, carbon nanotubes are also very conductive forphonons. Previous calculations predict that at room temperature, thermalconductivity of up to 6000 W/m K can be achieved with pure nanotubes,which is roughly twice as much as pure diamond. Nanotubes dispersedwithin a polymer matrix can thus provide thermally conductive resincompositions.

Carbon nanotubes have also been cited as having flame retardantproperties. Nanotubes dispersed within a polymer matrix can thus providematerials with fire proof properties.

Due to all of these properties, carbon nanotubes have been envisaged foruse in many applications in recent years (see P. Calvert “Potentialapplication of nanotubes” in Carbon Nanotubes, Editor T. W. Ebbeson,297, CRC, Boca Raton, Fla. 1997; T. W. Ebbeson, “Carbon Nanotubes”,Annu. Rev. Mater. Sci., 24, 235, 1994; Robert F. Service, “Super strongnanotubes show they are smart too”, Science, 281, 940, 1998; and B. I.Yakobson and R. E. Smalley, “Une technologie pour le troisièmemillénaire: les nanotubes”, La Recherche, 307, 50, 1998). However,currently the most promising line of research involves the mechanicalenhancement of polymers by using carbon nanotubes as reinforcingfillers.

Overall, it can be considered that there are four main requirements forthe carbon nanotubes to effectively reinforce the polymer and toincrease its conductivity and flame retardation properties: gooddispersion of the nanotubes in the polymer matrix, large aspect ratio ofthe nanotubes, efficient transfer of interfacial stress and alignment(Coleman et al., Carbon, 44, 2006, pp 1624-1652). Good dispersion can beconsidered to be the most important factor. The blends of carbonnanotubes and polymer must be homogeneous i.e. the nanotubes must beuniformly dispersed with the effect that each nanotube is individuallycoated with the polymer so that efficient load transfer to the nanotubenetwork can be achieved. Lack of homogeneity introduces stressconcentration centres i.e. weak points where there is, for instance, arelatively low concentration of nanotubes and a high concentration ofpolymer. Non-homogeneous nanotube-polymer composites therefore result inonly slight improvements in mechanical strength and little or noimprovement of electrical conductivity.

One of the current areas of research is the carbon nanotubereinforcement of thermoplastic polymers, in particular of commodityplastics such as polyolefins. However, as of yet melt processed blendsof polyolefins and carbon nanotubes have not produced the desiredresults. Dispersion of the nanotubes in the polyolefin matrix usingconventional techniques is poor due to the presence of Van der Waalsinteractions that favour the formation of carbon nanotube agglomerates.Other methods for blending, such as solution processing,surfactant-assisted processing, solution-evaporation methods withhigh-energy sonication and the like, which break up, to an extent, theagglomerates, have provided slightly better results. However, these aretime-, energy- and money-consuming processes. There is thus a need toimprove the results from melt processing, as it is the most preferredindustrial method for blending due to its speed, simplicity andcompatibility with standard industrial equipment. As a result of poordispersion from melt processing, the mechanical strengths and Young'smoduli of the nanotube-polyolefin composites are not increased to theextent expected and in certain cases are even decreased. Electricalconductivity of the resulting non-homogeneous composite is also lowerthan expected. Those procedures reported as providing homogeneousnanotube-polyolefin composites are often misleading, since in theseaggressive blending methods, the nanotubes are forced to break, therebylowering the aspect ratio and limiting the potential increase instiffness, strength and conductivity of the composite. Aggressiveblending can also result in the damaging of the surface of the carbonnanotubes, which also lowers stability and conductivity of thecomposite.

Tang et al. Carbon, 41, 2003, 2779-2785 report a MWNT/HDPE composite,which was compounded using a twin-screw extruder after preliminarymelt-mixing. From the figures in the relevant article it is seen thatwhile there are some individual nanotubes scattered in the matrix, mostof them are clumped together forming large aggregates. An attempt wasmade to co-feed the MWNTs directly into the extruder, but this techniquehad to be abandoned as the MWNTs had a tendency to stick to the hopperwalls. Overall mechanical properties improved, but not to the extentthat would be expected of carbon nanotubes. This is probably due to theproblems mentioned above regarding reduced aspect ratios and damagedsurfaces resulting from aggressive blending methods.

Lopez Manchado et al. Carbon 43, 2005, 1499-1505 reported a meltcompounded polypropylene and carbon nanotube composite. Lowconcentrations of nanotubes less than 1 wt % showed some improvement inmechanical properties. However, large aggregates of nanotubes werealready observed when only 1 wt % of carbon nanotubes was present, atwhich point stiffness and strength of the composite decreased.

EP 1 181 331 also discloses composites of carbon nanotubes andpolyolefins, whereby the mixture is stretched both in the molten stateand in the solidified state to increase alignment of the carbonnanotubes and thereby induce higher mechanical strengths in thecomposite therefrom. However, while stretching blends to orientate thecarbon nanotubes is a method suitable for fibre applications and tomaximize the mechanical properties of fibres, this may not be the casefor other applications. Aligned composites have an-isotropic mechanicalproperties that may need to be prevented in bulk samples.

EP 1 349 179 discloses partly purified carbon nanotubes i.e. carbonnanotubes that have not been partially oxidized during a purificationstep. It is shown that these nanotubes have a better dispersion inapolar polymers such as polyolefins. Oxidised nanotubes have alteredpolarities and hence a reduced affinity to apolar polymers, such aspolypropylene and polyethylene. However, in the figures one can stillsee the agglomerates of nanotubes in the nanoscale. There is thus a needto induce dispersion of nanotubes in the nanoscale in polyolefins.

Another method of enhancing miscibility of carbon nanotubes withpolyolefins has been by functionalizing the carbon nanotubes and/or thepolyolefins. Functionalisation is described in J. Chen et al., Science,282, 95-98, 1998; Y. Chen et al., J. Mater. Res., 13, 2423-2431, 1998;M. A. Hamon et al., Adv. Mater., 11, 834-840, 1999; A. Hiroki et al., J.Phys. Chem. B, 103, 8116-8121, 1999. The functionalisation can becarried out, for instance, by reaction with an alkylamine. It results ina better separation of the nanotubes in the polypropylene matrix therebyfavouring the dispersion in the polymer matrix. If the functionalisationis carried out in both the nanotubes and the polymer matrix it promotestheir covalent bonding, thereby improving the electrical and mechanicalproperties of the filled compound. However, functionalisation requires afurther reaction step, possibly even a further second step, if thepolymer is to be functionalised too. This makes the overall processcomplicated and costly and in general unsuitable for large-scaleindustrial production. Furthermore, functionalisation can change thephysical properties of the nanotubes, reducing their mechanical strengthand electrical conductivity.

It is hence an object of the invention to produce carbonnanotube-poly(hydroxy carboxylic acid) composites that are highlyhomogeneous.

It is therefore also an aim of the invention to enhance the dispersionof nanotubes in polyolefins in the nanoscale.

Furthermore, it is an aim of the invention to obtain a homogeneousnanotube-poly(hydroxy carboxylic acid) composite by melt processing.

Additionally, it is an aim of the invention to blend the carbonnanotubes with the polyolefin without requiring a functionalisation ormodification step respectively.

It is also an object of the invention to provide a resin with bettermechanical properties than polyolefins.

It is further an object of the invention to render electricallyinsulating compositions comprising polyolefins more electricallyconductive using carbon nanotubes.

Yet another aim of the invention is to increase the thermal conductivityof polyolefins with the effective dispersion of carbon nanotubestherein.

SUMMARY OF THE INVENTION

At least one of the objectives of the invention were met by providing aresin composition comprising a polyolefin, poly(hydroxy carboxylic acid)and nanotubes.

The invention also provides a process for preparing said resincomposition by:

-   -   i. blending a poly(hydroxy carboxylic acid) with nanotubes to        form a nanocomposite    -   ii. blending the nanocomposite with a polyolefin.

The invention also covers a process for preparing ananocompound-containing masterbatch by melt blending a poly(hydroxycarboxylic acid) with carbon nanotubes to form a composite.

Use of poly(hydroxy carboxylic acid) as a compatibiliser to blendnanotubes into polyolefins is also claimed.

DESCRIPTION OF THE INVENTION

Upon blending a poly(hydroxy carboxylic acid) with polyolefins, inparticular metallocene-catalysed polyolefins, the Applicant noted thathomogeneous blends could be achieved via simple melt blending withoutthe need of compatibilisers.

It was also noted that composites of carbon nanotubes and poly(hydroxycarboxylic acid)s were also homogeneous, with well-dispersed carbonnanotubes in the poly(hydroxy carboxylic acid) matrix.

The invention thus makes use of the compatibility of poly(hydroxycarboxylic acid)s with nanotubes and of the surprising compatibility ofpoly(hydroxy carboxylic acid)s with polyolefins, in particularmetallocene polyolefins.

First, a carbon nanotube-poly(hydroxy carboxylic acid) composite isprepared, which is to be used as a masterbatch for blending with thepolyolefin.

Carbon Nanotubes

As mentioned above, carbon nanotubes are used for reinforcement ofpolyolefins in the present invention. By carbon nanotubes it is meantcarbon-based tubes having a lattice structure related to the structureof Buckminsterfullerene (C₆₀). The nanotubes used in the composition canbe SWNT i.e. a one-atom thick sheet of graphite (called graphene) rolledup into a seamless hollow cylinder. MWNT can also be used, which areconcentric arrays of single-walled carbon nanotubes. Although termedcarbon nanotubes because of their diminutive dimensions, the carbonnanotubes used in the present invention need not necessarily havedimensions of the order of nanometers in size. The dimensions of thenanotubes can be much greater than this.

The diameter of the SWNT is preferably at most 50 nm, 25 nm, 20 nm, 18nm, 16 nm, 14 nm, 12 nm or 10 nm and at least 0.5 nm, 1 nm, 2 nm, 4 nm.Preferably the diameter of the SWNT is around 1 to 10 nm. The length ofa SWNT can be up to 5 cm, 2.5 cm or 1 cm and at least 1 μm, 10 μm or 100μm. Preferably the length of the SWNT is about 10 μm.

Multi walled carbon nanotubes (MWNT) can also be used according to theinvention. The diameter of the MWNT is preferably at most 100 nm, 75 nm,50 nm, 25 nm, 20 nm, 15 nm or 10 nm and at least 2 nm, 5 nm, 7 nm, 10nm. Preferably the diameter of the MWNT is 10 to 50 nm. The length of aMWNT can be up to 100 μm, 75 μm, 50 μm and at least 1 μm, 10 μm or 25μm. Preferably the length of the nanotube is about 10 to 50 μm.

To be used as effective reinforcement fillers, it is preferred that thenanotubes are endowed with a high aspect ratio, having a length/diameter(L/D) of 100 or more, preferably 10³ or more and most preferably 10⁴ ormore. Increasing the aspect ratio of the nanotubes (at constant nanotubevolume fraction and orientation) leads to enhanced strength andstiffness of the composite.

Carbon nanotubes can be produced by any method known in the art. Theycan be produced by the catalyst decomposition of hydrocarbons, atechnique that is called Catalytic Carbon Vapour Deposition (CCVD). Thismethod produces both SWNT and MWNT: the by-products are soot andencapsulated metal(s) nanoparticles. Other methods for producing carbonnanotubes include the arc-discharge method, the plasma decomposition ofhydrocarbons or the pyrolysis of selected polymer under selectedoxidative conditions. The starting hydrocarbons can be acetylene,ethylene, butane, propane, ethane, methane or any other gaseous orvolatile carbon-containing compound. The catalyst, if present, is eitherpure or dispersed on a support. The presence of a support greatlyimproves the selectivity of the catalysts but it contaminates the carbonnanotubes with support particles, in addition to the soot and amorphouscarbon produced during pyrolysis. Purification can remove theseby-products and impurities. This can be carried out according to thefollowing two steps:

-   -   1) the dissolution of the support particles, typically carried        out with an appropriate agent that depends upon the nature of        the support and    -   2) the removal of the pyrolytic carbon component, typically        based on either oxidation or reduction processes.

The term “carbon nanotubes” also includes the use of “functionalised”carbon nanotubes, as well as non-functionalised carbon nanotubes. Thesurface composition of the nanotubes can be modified in order to improvetheir dispersion in the polymer matrix and their linking properties:“functionalising” nanotubes is described for example in J. Chen et al.,Science, 282, 95-98, 1998; Y. Chen et al., J. Mater. Res., 13,2423-2431, 1998; M. A. Hamon et al., Adv. Mater., 11, 834-840, 1999; A.Hiroki et al., J. Phys. Chem. B, 103, 8116-8121, 1999. Thefunctionalisation can be carried out by reacting the carbon nanotubes,for example, with an alkylamine. It results in a better separation ofthe nanotubes in the polymer matrix thereby facilitating uniformdispersion within the polymer matrix. If the functionalisation iscarried out on both the nanotubes and the polymer, it promotes theircovalent bonding and miscibility, thereby improving the electrical andmechanical properties of the filled compound.

It should be noted that the use in the present invention of effectivelynon-continuous nanotubes (short in comparison to regular carbon fibres)rather than continuous fibres, allows access to standard processingtechniques as used for thermoplastics. These techniques permit highthroughput production and fabrication of high quality, complex shapedcomposites. Thus polymer composites comprising nanotubes can provide thebest of both worlds: high mechanical strength and ease of processing.

The Poly(Hydroxy Carboxylic Acid)

The poly(hydroxy carboxylic acid) can be any polymer wherein themonomers are derived from renewable resources and comprise at least onehydroxyl group and at least carboxyl group. The hydroxy carboxylic acidmonomer is preferably obtained from renewable resources such as corn andrice or other sugar- or starch-producing plants. Preferably thepoly(hydroxy carboxylic acid) used according to the invention isbiodegradable. The term “poly(hydroxy carboxylic acid)” includes homo-and copolymers herein.

The poly(hydroxy carboxylic acid) can be represented as in Formula I:

wherein

-   -   R9 is hydrogen or a branched or linear alkyl comprising from 1        to 12 carbon atoms;    -   R10 is optional and can be a branched, cyclic or linear alkylene        chains comprising from 1 to 12 carbon atoms; and    -   “r” represents the number of repeating units of R and is any        integer from 30 to 15000.

The monomeric repeating unit is not particularly limited, as long as itis aliphatic and has a hydroxyl residue and a carboxyl residue. Examplesof possible monomers include lactic acid, glycolic acid,3-hydroxybutyric acid, 4-hydroxybutyric acid, 4-hydroxyvaleric acid,5-hydroxyvaleric acid, 6-hydroxycaproic acid and the like.

The monomeric repeating unit may also be derived from a cyclic monomeror cyclic dimer of the respective aliphatic hydroxycarboxylic acid.Examples of these include lactide, glycolide, β-propiolactone,β-butyrlactone, γ-butyrolactone, γ-valerolactone, δ-valerolactone,ε-caprolactone and the like.

In the case of asymmetric carbon atoms within the hydroxy carboxylicacid unit, each of the D-form and the L-form as well as mixtures of bothmay be used. Racemic mixtures can also be used.

The term “poly(hydroxy carboxylic acid)” also includes blends of morethan one poly(hydroxy carboxylic acid).

The poly(hydroxy carboxylic acid) may optionally comprise one or morecomonomers.

The comonomer can be a second different hydroxycarboxylic acid asdefined above in Formula I. The weight percentage of eachhydroxycarboxylic acid is not particularly limited.

The comonomer can also comprise dibasic carboxylic acids and dihydricalcohols. These react together to form aliphatic esters, oligoesters orpolyesters as shown in Formula II having a free hydroxyl end group and afree carboxylic acid end group, capable of reacting with hydroxycarboxylic acids, such as lactic acid and polymers thereof.

wherein

-   -   R11 and R12 are branched or linear alkylenes comprising from 1        to 12 carbon atoms and can be the same or different;    -   “t” represents the number of repeating units T

These copolymers are also within the scope of the invention. The sum ofthe number of repeating units “r” (Formula I) and “t” (Formula II) isany integer from 30 to 15000. The weight percentages of each monomeri.e. the hydroxycarboxylic acid monomer and the aliphatic ester orpolyester comonomer of Formula II are not particularly limited.Preferably, the poly(hydroxy carboxylic acid) comprises at least 50 wt %of hydroxycarboxylic acid monomers and at most 50 wt % of aliphaticester, oligoester or polyester comonomers.

The dihydric alcohols and the dibasic acids that can be used in thealiphatic polyester unit as shown in Formula II are not particularlylimited. Examples of possible dihydric alcohols include ethylene glycol,diethylene glycol, triethyleneglycol, propylene glycol, dipropyleneglycol, 1,3-butanediol, 1,4-butanediol, 3-methyl-1,5-pentanediol,1,6-hexanediol, 1,7-octanediol, 1,9-nonanediol, neopentyl glycol,1,4-cyclohexanediol, isosorbide and 1,4-cyclohexane dimethanol andmixtures thereof.

Aliphatic dibasic acids include succinic acid, oxalic acid, malonicacid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaicacid, sebacic acid; undecanoic diacid, dodecanic diacid and3,3-dimethylpentanoic diacid, cyclic dicarboxylic acids such ascyclohexanedicarboxylic acid and mixtures thereof. The dibasic acidresidue in the hydroxy carboxylic acid copolymer can also be derivedfrom the equivalent diacylchlorides or diesters of the aliphatic dibasicacids.

In the case of asymmetric carbon atoms within the dihydric alcohol orthe dibasic acid, each of the D-form and the L-form as well as mixturesof both may be used. Racemic mixtures can also be used.

The copolymer can be an alternating, periodic, random, statistical orblock copolymer.

Polymerisation can be carried out according to any method known in theart for polymerising hydroxy carboxylic acids. Polymerisation of hydroxycarboxylic acids and their cyclic dimmers is carried out bypolycondensation or ring-opening polymerisation.

Copolymerisation of hydroxycarboxylic acids can be carried out accordingto any method known in the art. The hydroxycarboxylic acid can bepolymerised separately prior to copolymerisation with the comonomer orboth can be polymerised simultaneously.

In general, the poly(hydroxy carboxylic acid), homo- or copolymer(copolymerised with a second different hydroxy carboxylic acid or withan aliphatic ester or polyester as described above), may also comprisebranching agents. These poly(hydroxy carboxylic acid)s can have abranched, star or three-dimensional network structure. The branchingagent is not limited so long as it comprises at least three hydroxylgroups and/or at least three carboxyl groups. The branching agent can beadded during polymerisation. Examples include polymers such aspolysaccharides, in particular cellulose, starch, amylopectin, dextrin,dextran, glycogen, pectin, chitin, chitosan and derivates thereof. Otherexamples include aliphatic polyhydric alcohols such as glycerine,pentaerythritol, dipentaerythritol, trimethylolethane,trimethylolpropane, xylitol, inositol and the like. Yet another exampleof a branching agent is an aliphatic polybasic acid. Such acids includecyclohexanehexacarboxylic acid, butane-1,2,3,4-tetracarboxylic acid,1,3,5-pentane-tricarboxylic acid, 1,1,2-ethanetricarboxylic acid and thelike.

The total molecular weight of the poly(hydroxy carboxylic acid) dependson the desired mechanical and thermal properties and mouldability of thenanotube composite and of the final resin composition. It is preferablyfrom 5,000 to 1,000,000 g/mol, more preferably from 10,000 to 500,000g/mol and even more preferably from 35,000 to 200,000 g/mol. Mostpreferably the total molecular weight of the polymer is from 40,000 to100,000 g/mol.

The molecular weight distribution is generally monomodal. However, inthe case of mixtures of two or more fractions of poly(hydroxy carboxylicacid)s of different weight average molecular weight and/or of differenttype, the molecular weight distribution can also be multimodal e.g. bi-or trimodal.

From a standpoint of availability and transparency, the poly(hydroxycarboxylic acid) is preferably a polylactic acid (PLA). Preferably thepolylactic acid is a homopolymer obtained either directly from lacticacid or from lactide, preferably from lactide.

In the past few years, the general public has become increasinglyapprehensive of the impact man-made waste has on the environment. Hencethere is a growing interest in developing novel biodegradable (andpreferably compostable) plastics from renewable resources.

One particularly interesting candidate for this task are poly(hydroxycarboxylic acid)s, in particular polylactic acid, now commerciallyavailable on a relatively large scale. The lactic acid is obtained fromplants such as corn and rice or other sugar- or starch-producing plants.Not only is PLA obtainable from renewable materials, it is also easilycompostable. For these reasons, there is significant interest in usingPLA as a substitute in applications, where petroleum-basedthermoplastics have conventionally been used.

Unfortunately, PLA used on its own does not have the same advantageousproperties as conventional plastics do. In particular PLA hasperformance problems related to heat resistance, brittleness and limitedflexibility, resulting in poor mechanical strength. On the other hand,polyolefins, in particular polypropylene, have much better mechanicalproperties: It has been attempted to combine these properties byblending PLA with polyolefins to obtain a resin that is at leastpartially biodegradable, but still has acceptable mechanical properties.However, up until now it was assumed that it would be difficult, evenimpossible, to obtain homogeneous PLA and polyolefin blends, due to thedifferences in polarity and molecular weight distribution. In the past,compatibilising agents were used. However, this requires an additionalindustrial step, as well as specific conditions during extrusion.Furthermore, undesirable by-products are created when addingcompatibilising agents. Thus both the compatibilising agent and theby-products change the properties of the desired end product, be it afilm, fibre or moulded object.

Using biodegradable poly(hydroxy carboxylic acid)s to disperse carbonnanotubes into polyolefins thus has the added benefit of providing aresin that is at least partially biodegradable and/or partiallyobtainable from renewable resources.

Thus, preferably the poly(hydroxy carboxylic acid) that is selected isbiodegradable, for example polylactic acid. Biodegradability is hereindefined as provided by the standard EN 13432:2000. In order forpackaging material to be biodegradable it must have a lifecycle, whichcan be described as follows:

-   -   a period of storage and/or use starting from time t₀, which is        the moment the material comes off the production line;    -   a period of disintegration starting at time t₁, during which the        polymer begins to significantly chemically disintegrate e.g. via        the hydrolysis of ester bonds;    -   a period of biodegradation, during which the partly hydrolysed        polymer biologically degrades as a result of the action of        bacteria and micro organisms;

It is important to make the distinction between degradable,biodegradable and compostable as often these terms are usedinterchangeably. In addition to the above, a compostable plastic is“capable of undergoing biological decomposition in a compost site aspart of an available program, such that the plastic is not visuallydistinguishable and breaks down to carbon dioxide, water, inorganiccompounds, and biomass, at a rate consistent with known compostablematerials (e.g. cellulose) and leaves no toxic residue” (ASTM). On theother hand a degradable plastic is one which is merely chemicallychanged i.e. there is no requirement for the plastic to be biologicallydegraded by microorganisms. Therefore, a degradable plastic is notnecessarily biodegradable and a biodegradable plastic is not necessarilycompostable (that is, it breaks down too slowly and/or leaves toxicresidue).

In particular, the EN 13432:2000 standard for compostability has thefollowing main features:

-   -   Disintegration is measured by sieving the material to determine        the biodegraded size. To be considered compostable, less than        10% of the material should be larger than 2 mm in size.    -   Biodegradability is determined by measuring the amount of carbon        dioxide produced over a certain time period by the biodegrading        plastic. To be considered compostable, it must be 90%        biodegraded within 90 days.    -   Eco-toxicity is measured by determining whether the        concentration of heavy metals is below the limits set by the        standard and by testing plant growth by mixing the compost with        soil in different concentrations and comparing it with        controlled compost.        Composite Processing of Poly(Hydroxy Carboxylic Acid) and        Nanotubes

The poly(hydroxy carboxylic acid) and the carbon nanotube are blendedtogether to form a nanotube-polymer composite. This composite can thenbe used as a masterbatch to be added to a polyolefin and to introducecarbon nanotubes into the polyolefin composition more homogeneously thandirect addition of the carbon nanotube to the polyolefin.

In particular, the Applicant has observed that blends of carbonnanotubes and poly(hydroxy carboxylic acid)s are surprisinglyhomogeneous. It appears that the polarity of the nanotubes is moresimilar to poly(hydroxy carboxylic acid)s than to polyolefins.Therefore, the carbon nanotube-poly(hydroxy carboxylic acid) compositesare more homogeneous than if the carbon nanotube were blended directlyinto the polyolefin.

The method of composite processing i.e. blending is not particularlylimited and can be carried out according to any known method in the art.One example of composite processing is solution processing whereby thenanotubes and the poly(hydroxy carboxylic acid) are mixed in a suitablesolvent before evaporating said solvent to obtain the composite. Mixingcan occur for example by magnetic stirring, shear mixing, refluxing, orultrasonication. Another method that can be used to blend the nanotubesinto the polymer is in situ polymerisation. In this casehydroxycarboxylic acids (or cyclic dimers and trimers thereof) arepolymerised in the presence of either carbon nanotubes and catalyst, orcarbon nanotubes acting as a catalytic support for the polymerisationcatalyst. It is also possible to dry blend the nanotubes and thepolymer. Dry blending can also be carried out prior to the meltprocessing stage.

However the preferred method for composite processing is meltprocessing. This technique takes advantage of the fact thatthermoplastics soften when heated above their glass transitiontemperature (for polymers that are amorphous at room temperature) orabove their melt temperature (for polymers that are semi-crystalline atroom temperature). Melt processing is fast and simple and makes use ofstandard equipment of the thermoplastics industry. The components can bemelt blended by shear mixing in a batch process such as in a Banbury orBrabender Mixer or in a continuous process, such as in an extruder e.g.a twin screw extruder. During melt blending, the temperature in theblender will generally be in the range between the highest melting pointof poly(hydroxy carboxylic acid) employed and up to about 150° C. abovesuch melting point, preferably between such melting point and up to 100°C. above such melting point.

The time required for the blending can vary broadly and depends on themethod of blending employed. The time required is the time sufficient tothoroughly mix the components. Generally, the individual polymers areblended for a time of about 10 seconds to about 60 minutes, preferablyto about 45 minutes, more preferably to about 30 minutes.

The proportion of carbon nanotubes added to a given quantity ofpoly(hydroxy carboxylic acid) is not particularly limited. The carbonnanotubes are present at up to 99 wt % of the composites, preferably upto 75 wt %, more preferably up to 50 wt %, even more preferably up to 25wt %, more preferably than that up to 20 wt %. It is most preferred thatat most 5 wt % of nanotubes are added. A very small quantity ofnanotubes is capable of beneficially affecting the properties of apolymer, such that very small quantities can be used, depending on theintended use of the polymer. However, for most applications it ispreferred that 0.1 wt % of nanotubes or greater is added.

The proportion of poly(hydroxy carboxylic acid) is not particularlylimited. It can range from 1 to 99 wt % of the total composite.Preferably, the composite comprises at least 25 wt % of the poly(hydroxycarboxylic acid), more preferably at least 50 wt %, even more preferablyat least 75 wt % and more preferably than that at least 80 wt % of thepoly(hydroxy carboxylic acid). Most preferably, the composite comprisesat least 95 wt % of the poly(hydroxy carboxylic acid).

Any other additive can be included in the composite masterbatch. Thusadditives such as pigments, carbon black, anti-oxidants, UV-protectors,lubricants, anti-acid compounds, peroxides, grafting agents andnucleating agents can be included. However, they may alternatively beadded whilst blending the nanotube composite masterbatch with thepolyolefin or they be added to the polyolefin prior to its blending withthe nanotube composite.

The Polyolefin

Once the nanotube composite masterbatch has been prepared, it can beblended into a resin comprising one or more polyolefins without the needof any compatibilisers.

The polyolefin can be any polymer of α-olefins. The term “polyolefin”herein includes homo- and copolymers of α-olefins. The α-olefin is any1-alkylene comprising from 2 to 12 carbon atoms, for example, ethylene,propylene, 1-butene, 1-pentene and 1-hexene. When the polyolefin is apolymer of an olefin having 3 or more carbon atoms, such aspolypropylene, the polyolefin may be atactic, isotactic or syndiotactic.

If the polyolefin is a copolymer, the comonomer can be any α-olefin i.e.any 1-alkylene comprising from 2 to 12 carbon atoms, but different fromthe main monomer. In certain cases, the comonomer can also be anyfunctionalised compound that comprises a vinyl group. These kind ofvinyl-containing comonomers comprise from 2 to 12 carbon atoms andinclude, for example, vinyl acetate, acrylic acids and acrylates. Thecopolymer can be an alternating, periodic, random, statistical or blockcopolymer.

The term polyolefin herein also includes blends of two or morepolyolefins as defined above.

Preferably, the polyolefin used in the resin composition of theinvention is a homo- or copolymer of ethylene or propylene.

The α-olefins can be polymerised either at high pressure or at lowpressure. When polymerising at high pressure, in particular ethylene, nocatalyst is required as the polymerisation occurs via a radicalmechanism. The polymerisation of ethylene at high pressure can beinitiated using an initiator, for example, a peroxide. Ethylenepolymerised at high pressure is known as low density polyethylene(LDPE). It has a density of between 0.910 and 0.940 g/cm³ due to thepresence of a high degree of long and short chain branching. It hasunique flow properties, allowing it to be easily processed. However, thecrystal structure of LDPE is not packed very tightly and the inter- andintramolecular forces are weak. Therefore, mechanical properties such astensile strength, environmental stress crack resistance (ESCR) and tearresistance are particularly low in LDPE. However by blending LDPE withcarbon nanotube-containing poly(hydroxy carboxylic acid)s, themechanical properties of LDPE are greatly improved, without losing anyof its processing advantages.

Preferably the ethylene is polymerised at high pressure with acomonomer, wherein the comonomer is one of the vinyl-containingcompounds described above, for example, vinyl acetate, acrylic acids andacrylates. These comonomers impart on the LDPE polar properties. Thusthe LDPE copolymer is more compatible with the poly(hydroxy carboxylicacid)-nanotube composite and the two components can be easily mixed toform a homogeneous blend. No compatibiliser is required for thispurpose. Most preferably, the copolymer is a ethylene-vinyl acetatepolymer, the comonomer being vinyl acetate.

The relative amount of comonomer in the high pressure ethylene copolymeris not particularly limited. Preferably, the comonomer content of highpressure ethylene copolymers does not exceed 30 wt % of the ethylenecopolymer. More preferably it does exceed 20 wt % and most preferably itis at most 10 wt %.

Alternatively, any type of low-pressure polymerised polyolefin,catalysed by any known appropriate means in the art, can be used in theresin composition according to the invention. Examples of suitablecatalysts include single site catalysts (in particular metallocenecatalysts), Ziegler-Natta catalysts, and chromium catalysts. Ifrequired, more than one catalyst of the same or different type can beused, either simultaneously in one reactor, in two parallel reactors orin two reactors connected to each other in series, to obtain multimodalor broader molecular weight distributions.

Examples of suitable catalysts for polymerising ethylene, in particular,include single site catalysts (in particular metallocene catalysts),Ziegler-Natta catalysts, and chromium catalysts. However any othercatalyst known in the art can be used too. Low-pressure polymerisedethylene is more linear than LDPE, having low concentrations of longchain branching, giving it stronger intermolecular forces and highertensile strength than LDPE. Low-pressure polymerised ethylene can bebroadly categorised as linear low density (LLDPE), medium density (MDPE)and high density (HDPE) polyethylene, the density being mainly regulatedby the relative amount of comonomer added; the more comonomer added, thehigher the degree of short chain branching and the lower the density.Preferably, the comonomer is polypropylene, 1-butene, 1-pentene or1-hexene.

Examples of suitable catalysts for polymerising propylene includeZiegler-Natta and single site catalysts (in particular metallocenecatalysts). However any other catalyst known in the art can be used too.The polypropylene can be syndiotactic, isotactic or atactic. Isotacticpolypropylenes can be obtained using Ziegler-Natta catalysts orappropriate single site catalysts (in particular metallocene catalysts).Syndiotactic and atactic polypropylenes are obtainable using appropriatesingle site catalysts (in particular metallocene catalysts). Isotacticpolypropylene is generally selected.

The overall properties of the polyolefin are dependent on the method andtype of catalyst used. Single-site catalysed polyolefins, in particularmetallocene-catalysed polyolefins, are the preferred polyolefins for thepurposes of this invention. It has been found that poly(hydroxycarboxylic acid)s are more miscible with single-site catalysedpolyolefins, in particular metallocene-catalysed polyolefins, than thoseblended with Ziegler-Natta or chromium catalysed polyolefins. Blends ofsingle-site catalysed polyolefins, in particular metallocene-catalysedpolyolefins, with poly(hydroxy carboxylic acid)s are homogeneous and donot require any compatibilisation.

Compared to non-metallocene catalysed polyolefins, single-site catalysedpolyolefins, in particular metallocene-catalysed polyolefins, have amuch narrower molecular weight distribution. Preferably, the molecularweight distribution is of from 1 to 10, preferably from 1 to 7, morepreferably from 1 to 5, most preferably from 1 to 4. The narrowmolecular weight distribution is compatible with the similarly narrowmolecular weight distribution of poly(hydroxy carboxylic acid)s.

Without wishing to be bound by theory, it is thought that the molecularstructure of single-site catalysed polyolefins, in particularmetallocene-catalysed polyolefins, induces a better compatibility withpoly(hydroxy carboxylic acid)s as well. These polyolefins show no orvery little long chain branching. The incorporation of comonomers occursvery regularly along the polyolefin backbone resulting in a highlyuniform distribution of comonomers i.e. regular short chain branching.This effect (known as very narrow short chain branching distributions(SCBD)) in polyolefins is specific to single-site catalysed polyolefins,in particular metallocene-catalysed polyolefins. As a result, during thecrystallization from the melt, very small crystallites are formedthroughout the material, thus providing excellent optical clarity.Ziegler-Natta and chromium-catalysed polyolefins on the other hand, havea poor and very random comonomer incorporation, therefore duringcrystallisation a broad distribution of different sizes of crystallitesoccurs, resulting in high haze values.

The Applicant believes, without wishing to be bound by theory, thatsince the molecular architecture of poly(hydroxy carboxylic acid)s issimilar to that of single site catalysts (in particular metallocenecatalysts), i.e. narrow molecular weight distribution, no long chainbranching and narrow short chain branching distributions (if shortchains are present at all), poly(hydroxy carboxylic acid)s are thereforemore compatible with single-site catalysed polyolefins, in particularmetallocene-catalysed polyolefins, than with other polyolefins.

The polyolefin resin may also contain additives such as pigments, carbonblack, anti-oxidants, UV-protectors, lubricants, anti-acid compounds,peroxides, grafting agents and nucleating agents can already beincluded. However, they may alternatively be added to the nanotubecomposite masterbatch prior to blending with the polyolefin. They mayalso be added during blending of the two components of the resincomposition according to the invention.

Blending of the Nanotube-Polymer Composite Masterbatch with thePolyolefin.

The blending of the nanotube-poly(hydroxy carboxylic acid) compositewith the polyolefin can be carried out according to any physicalblending method known in the art. This can be, for instance, wetblending or melt blending. The blending conditions depend upon theblending technique and polyolefin involved. Depending on the method, thepolyolefin and the nanotube composite can be in any appropriate form,for example, fluff, powder, granulate, pellet, solution, slurry, and/oremulsion.

If dry blending of the polymer is employed, the dry blending conditionsmay include temperatures from room temperature up to just under themelting temperature of the polymer, and blending times in the range of afew seconds to hours. The components are dry blended prior to meltblending.

Melt processing is fast and simple and makes use of standard equipmentof the thermoplastics industry. The components can be melt blended in abatch process such as with a Banbury or Brabender Mixer or in acontinuous process, such as with a typical extruder e.g. a twin screwextruder. During melt blending, the temperature at which the polymersare combined in the blender will generally be in the range between thehighest melting point of the polymers employed and up to about 150° C.above such melting point, preferably between such melting point and upto 100° C. above such melting point. The time required for the meltblending can vary broadly and depends on the method of blendingemployed. The time required is the time sufficient to thoroughly mix thecomponents. Generally, the individual polymers are blended for a time ofabout 10 seconds to about 60 minutes, preferably to about 45 minutes,more preferably to about 30 minutes.

The components can also be wet blended whereby at least one of thecomponents is in solution or slurry form. If solution blending methodsare employed, the blending temperature will generally be 25° C. to 50°C. above the cloud point of the solution involved. The solvent ordiluent is then removed by evaporation to leave behind a homogeneousblend of poly(hydroxy carboxylic acid) and polyolefin with carbonnanotubes dispersed throughout the mixture.

The resin composition comprises from 1 to 50 wt % of the carbonnanotube-poly(hydroxy carboxylic acid) composite, preferably from 1 to40 wt %, more preferably from 1 to 30 wt % and most preferably from 1 to20 wt %. The resin composition comprises from 1 to 99 wt % of thepolyolefin, preferably from 25 to 99 wt %, more preferably from 50 to 99wt %, even more preferably from 75 to 99 wt % and most preferably from80 to 99 wt %.

Preferably, carbon nanotubes make up at least 0.05 wt % of the totalresin composition. Preferably, the carbon nanotube content of the totalresin composition does not exceed 10 wt %, more preferably it doesexceed 5 wt % and most preferably it does exceed 3 wt %.

Preferably, the resin composition essentially consists of a polyolefin,carbon nanotubes and poly(hydroxy carboxylic acid).

Due to the improved mechanical properties of the polyolefin in the resincomposition, and also the improved electrical and thermal conductivity,as well as partial biodegradability of the resin composition, it issuitable for a wide variety of applications.

The improved mechanical properties make the resin composition suitablefor fibre applications. The fibres of the invention have higherstiffness, increased tensile strength, higher tenacity, better energyabsorption capabilities and very good strain at break. Hydrophilicity ofthe polyolefin-containing fibre is also increased due to the presence ofthe polar poly(hydroxy carboxylic acid) component. The fibres can beproduced on an industrial scale as multi-filament yarns, but stillhaving the advantageous properties of the monofilament. Examples ofarticles made from the fibre comprising the resin composition of theinvention are ropes, nets and cables. The light fibres having improvedmechanical strength can also be used in anti-ballistic composites tomake light protective clothing.

The resin composition can also be transformed into a film with improvedprintability, better surface tension, increased thermal and highfrequency sealability, improved stiffness and enhanced breathability.The film also has good barrier properties against atmospheric gases, inparticular oxygen and nitrogen. The resin composition can also be usedto manufacture pouches, for example, for medical applications.

The composition is also suitable for typical injection, extrusion andstretch blow moulding applications, but also thermoforming, foaming androtomoulding. The articles made according to these processes can bemono- or multilayer, at least one of the layers comprising the resincomposition of the invention.

The resin can also be used in applications that require dissipation ofstatic electricity e.g. electrically dissipative parts for automotiveapplications, conductive video disks, conductive textiles, stand shieldsfor wires and cables, cable jacketing, hospital tiles, computer tapes ormine belting. With a higher content of nanotubes in the resin,electrical conductivity is further enhanced and allows for otherapplications such as, for instance, parts that can be electrostaticallypainted for the automotive industry.

The resins also exhibits a flame retardant effect as measured bythermogravimetric analysis (TGA) and cone calorimetry tests. This effectis more pronounced when nanotubes are used in combination with classicalflame retardants, such as ATH (aluminium trihydrate) and magnesiumhydroxide, due to the presence of synergistic effects between bothcompounds.

EXAMPLES Example A

Metallocene-catalysed polypropylene (MR2001 from Total Petrochemicals®)having MFR of 25 dg/min is extruded in a Haake® Minilab twin-screwextruder. Temperature is set at 200° C. and the resin is recirculatedfor 2 minutes before extrusion. Fibres from the obtained material arethen drawn using a Ceast® laboratory equipment. Mechanical tests arecarried out on the material.

Example B

Metallocene-catalysed polypropylene (MR2001 from Total Petrochemicals®)having MFR of 25 dg/min is mixed with carbon nanotubes (GraphistrengthC100 from Arkema®) in a Haake® Minilab twin-screw extruder. Temperatureis set at 200° C. and the mixture is recirculated for 2 minutes beforeextrusion. Fibres from the obtained material are then drawn using aCeast® laboratory equipment. Mechanical properties are improved ascompared to Example A.

Example C According to the Invention

PLA from Unitika is mixed with carbon nanotubes (Graphistrength C100from Arkema®) in a Haake® Minilab twin-screw extruder. Temperature isset at 200° C. and the mixture is recirculated for 2 minutes beforeextrusion. The extrudate is then blended with metallocene-catalysedpolypropylene (MR2001 from Total Petrochemicals®) using the sameequipment and a temperature of 200° C. Fibres from the obtained materialare then drawn using a Ceast® laboratory equipment. Mechanicalproperties are further improved as compared to Example B.

The invention claimed is:
 1. A process for preparing a resin compositioncomprising: blending a poly(hydroxy carboxylic acid) with carbonnanotubes to form a composite, wherein the blending of the poly(hydroxycarboxylic acid) with the carbon nanotubes comprises solutionprocessing, dry blending, dry blending and melt blending, or meltblending; and blending the composite with a polyolefin to form the resincomposition.
 2. The process of claim 1, wherein the polyolefin wasprepared with a single-site catalyst.
 3. The process of claim 1, whereinthe poly(hydroxy carboxylic acid) is polylactic acid.
 4. The process ofclaim 1, wherein the poly(hydroxy carboxylic acid) is melt blended withcarbon nanotubes.
 5. A resin composition comprising a polyolefin and acomposite comprising carbon nanotubes and poly(hydroxy carboxylic acid),wherein the poly(hydroxy carboxylic acid) and the carbon nanotubes areblended via solution processing, dry blending, dry blending and meltblending, or melt blending.
 6. The resin composition of claim 5, whereinthe polyolefin was prepared with a single-site catalyst.
 7. The resincomposition of claim 5, wherein the poly(hydroxy carboxylic acid) ispolylactic acid.
 8. The resin composition of claim 5, wherein the resincomposition comprises at most 10% by weight of carbon nanotubes based ona total weight of the resin composition.
 9. The resin composition ofclaim 5, wherein the polyolefin is selected from polypropylene andpolyethylene homo- and copolymers.
 10. The resin composition of claim 5,wherein the carbon nanotubes and the poly(hydroxy carboxylic acid) areblended to form the composite prior to blending the composite with thepolyolefin.
 11. The resin composition of claim 5, wherein the carbonnanotubes are single walled carbon nanotubes having: a diameter of atleast 0.5 nanometers and at most 50 nanometers; a length of at least 1microns and at most 5 centimeters; and a length/diameter aspect ratio ofat least
 100. 12. The resin composition of claim 5, wherein the carbonnanotubes are multi-walled carbon nanotubes having: a diameter of atleast 2 nanometers and at most 100 nanometers; a length of at least 1micron and at most 100 microns; and a length/diameter aspect ratio of atleast
 100. 13. The resin composition of claim 5, wherein the carbonnanotubes are non-functionalised carbon nanotubes.
 14. The resincomposition of claim 5, wherein the resin composition comprises from 1to 50 wt % of the composite.
 15. The resin composition of claim 5,wherein the resin composition comprises from 1 to 99 wt % of thepolyolefin.
 16. The resin composition of claim 5, wherein the compositecomprises up to 99 wt % of the carbon nanotubes and from 1 wt % to 99 wt% of the poly(hydroxy carboxylic acid).
 17. The process of claim 1,wherein the polyolefin is melt blended with the composite.
 18. Theprocess of claim 1, wherein the formed composite is a homogenouscomposite comprising the carbon nanotubes dispersed in a matrix of thepoly(hydroxy carboxylic acid); and wherein the formed resin compositionis a homogenous blend of the polyolefin and the poly(hydroxy carboxylicacid) with the carbon nanotubes dispersed throughout the homogenousblend.
 19. A process for preparing a resin composition comprising:polymerizing hydroxy carboxylic acid monomers to form poly(hydroxycarboxylic acid); blending the poly(hydroxy carboxylic acid) with carbonnanotubes to form a composite; and blending the composite with apolyolefin to form the resin composition, wherein the compositecomprises the carbon nanotubes dispersed within the poly(hydroxycarboxylic acid).
 20. The process of claim 19, wherein the blending ofthe poly(hydroxy carboxylic acid) with the carbon nanotubes comprisessolution processing, dry blending, dry blending and melt blending, ormelt blending.